This invention is a process for the alkylation of an aromatic hydrocarbon which comprises contacting the aromatic hydrocarbon with a stoichiometric or excess amount of a C.sub.2 to C.sub.4 olefin alkylating agent and in the presence of a catalyst comprising zeolite beta. This is especially useful for the production of a mixture of ethylbenzene and diethylbenzene by the reaction of dilute ethylene with dilute benzene. It can be carried out in a fixed bed reactor or more preferably in a catalytic distillation reactor.
The known processes for the manufacture of ethylbenzene use the Friedel-Crafts reaction of alkylation of benzene by ethylene. Similarly, Friedel-Crafts reaction of alkylation of benzene by propylene is used to manufacture cumene.
The catalysts for this reaction are typically Bronsted or Lewis acids, including aluminum chloride, boron trifluoride deposited on alumina, or zeolites used in liquid or gas phase.
One of the difficulties encountered in this reaction is for example when ethylene is used as the alkylating agent that the ethylbenzene formed is more reactive than benzene with respect to ethylene, which leads to the production of diethylbenzenes, which are themselves more reactive than ethylbenzene, and therefore have a tendency to form triethylbenzenes. To limit these polyalkylation reactions, the prior art teaches the use of a large excess of benzene with respect to the ethylene at the entry of the alkylation reactors. Thus, the benzene/ethylene molar ratio is generally between 2 and 2.5 for the processes using aluminum chloride, and the ratio may even reach a value between 8 and 16 for processes using zeolites in the gas phase. In spite of the use of an excess of benzene with respect to the ethylene to minimize the formation of polyethylbenzenes, such formation cannot be completely avoided.
It is becoming increasingly desirable to be able to economically remove the majoity of the benzene from streams being blended into gasoline to meet environmental regulations. Prior art describes means to accomplish this by alkylating, the benzene. Such approaches while successfully reducing the benzene contained in the gasoline by converting it to higher boiling alkyl benzenes, typically do not remove the aromatic rings. Therefore, total aromatic content in the gasoline remains essentially unchanged. Environmental regulations for gasoline and distillate fuels are increasingly limiting both the benzene and total aromatic content. Therefore from an environmental standpoint it is more desirable to remove the benzene ring from the gasoline.
It is also economically attractive to make use of the benzene in such streams as a feedstock in processes that make high-valued petrochemicals such as ethylbenzene and cumene instead of requiring a purified benzene stream as the feedstock, as is now practiced in the industry.
U.S. Pat. No. 4,891,458 discloses: a process for the alkylation of an aromatic hydrocarbon which comprises contacting the aromatic hydrocarbon with a C.sub.2 to C.sub.4 olefin under at least partial liquid phase conditions and in the presence of a catalyst comprising zeolite beta. (column 2, lines 33-39). This same patent further discloses, "When alkylation is the process conducted according to this invention, reaction conditions are as follows. The aromatic hydrocarbon feed should be present in stoichiometric excess. It is preferred that the molar ratio of aromatics to olefins be at least about four to one (4:1) to prevent rapid catalyst fouling." (column 5, lines 24-29). In U.S. Pat. No. 5,081,323, a continuation of U.S. Pat. No. 4,891,458, feeding a part of the aromatic stream between reactor beds is disclosed.
Published EP-A-571,701 discloses a process for alkylating a hydrogenated dilute benzene with a dilute olefin stream. The dilute benzene is first hydrogenated in order to remove C.sub.5 -C.sub.7 olefins. Zeolite beta is specifically disclosed as a suitable catalyst. The molar ratio of aromatics to olefins is required to be at least about three to one (3:1). Further, the aromatic hydrocarbon feed should be present in a stoichimetric excess, and it is preferred that the molar ratio of aromatics to olefins be at least 3:1 to prevent catalyst fouling (page 6. lines 42-44).
In the prior art thus far discussed, a stoichimetric excess of benzene is employed, this can easily be achieved when commercially pure benzene is used as a feedstock. In this instance as is well known in the arts, the unreacted benzene is recovered downstream by distillation and merely recycled back to the reactor. This maintains the stoichimetric excess of benzene in the alkylation reactor feed and achieves high ultimate conversion of benzene. However, when the feed stream is dilute in benzene, high conversion of the benzene is not so easily achievable, when a stoichiometric excess is required by the process. In this latter situation the unreacted benzene will be diluted with materials not easily separated by distillation, and therefore would rapidly build up if the stream containing them were recycled to the alkylation reactor. A common approach to avoid this undesirable result is to purge a significant fraction of the steam containing the unreacted benzene, which necessarily results in a relatively low ultimate conversion of the benzene originally in the feedstream.
For the foregoing reasons there is a need for a process which is able to remove the benzene from hydrocarbon streams in the gasoline boiling range, which at the same time does not require the use of excess benzene, achieves a high level of benzene removal, can use dilute ethylene or propylene as an alkylating agent, and allows for the recovery of high valued petrochemical products such as ethylbenzene or cumene.